In recent years, demands for making much narrower intervals or pitches between circuit patterns on a circuit board increase to make electronic devices much smaller and to save resources and energy for their manufacturing. Accordingly, circuit patterns are formed on the both sides of a circuit board, which are electrically connected by a through hole of a conductive layer. Although the through hole is conventionally formed by drilling, recently it is formed during the injection-molding process for the circuit board itself (Patent Documents 1, 2, and 3).
Patent Document 1: JP unexamined patent application publication No. 61-239694
Patent Document 2: JP unexamined patent application publication No. 63-128181
Patent Document 3: U.S. Pat. No. 4,424,095
Structure of a conventional solid or three-dimensional circuit board 11 will be described with reference to FIGS. 5 to 7. A circuit board 21 is provided with circuit patterns 31, 41 on the both sides thereof, and the circuit patterns are electrically connected by through holes 51. The diameter of the through hole is in several-ten-micron order. The circuit board having those through holes is formed by injection molding with a molding die having a circular-cylindrical pin implanted. The diameter of the through hole is several-ten-micron order, i.e. 30 to 50 microns. The shape of the through hole 51 is cylindrical and has the same diameter in any points of its longitudinal direction. Ratio of the thickness of the circuit board 21 to the internal diameter of the through hole 51 or aspect ratio is limited to about five to one (5:1). Namely, if the aspect ratio falls within 5:1, electroless plating can deposit, however, if the aspect ratio exceeds 5:1, deposit by electroless plating is either difficult or impossible.